When functioning properly, a heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout a circulatory system. This intrinsic rhythm is a function of intrinsic signals generated by the sinoatrial node, or SA node, located in the upper right atrium. The SA node periodically depolarizes, which in turn causes the atrial heart tissue to depolarize such that right and left atria contract as the depolarization travels through the atrial heart tissue. The atrial depolarization signal is also received by the atrioventricular node, or AV node, which, in turn, e.g., after a brief delay of approximately 0.12 seconds, triggers a subsequent ventricular depolarization signal that travels through and depolarizes the ventricular heart tissue causing the right and left ventricles to contract.
Some patients, however, have irregular cardiac rhythms, referred to as cardiac arrhythmias. Cardiac arrhythmias result in diminished blood circulation because of diminished cardiac output. Atrial fibrillation is a common cardiac arrhythmia that reduces the pumping efficiency of the heart. Atrial fibrillation is characterized by rapid, irregular, uncoordinated depolarizations of the atria. These depolarizations may not originate from the SA node, but may instead originate from an arrhythmogenic substrate, such as an ectopic focus, within the atrial heart tissue. The reduced pumping efficiency due to atrial fibrillation requires the ventricle to work harder, which is particularly undesirable in sick patients that cannot tolerate additional stress. As a result of atrial fibrillation, patients must typically limit activity and exercise.
Other types of cardiac arrhythmias originate in the ventricles. Ventricular arrhythmias may compromise pumping efficiency even more drastically than atrial arrhythmias. Some ventricular arrhythmias are treated using defibrillation and/or cardioversion therapy.
In some cases, an atrial fibrillation or other atrial tachyarrhythmia induces rapid and irregular ventricular heart rhythms. Rapid and/or irregular atrial depolarization signals associated with atrial tachyarrhythmia are received by the AV node, and may be conducted to the ventricles. Such induced ventricular arrhythmias compromise pumping efficiency even more drastically than atrial arrhythmias, and are referred to as supraventricular tachycardia (SVT). Although SVT is not usually treatable by delivery of defibrillation and/or cardioversion therapy to the ventricles, in some cases a medical device misclassifies an SVT as a ventricular tachyarrhythmia, and inappropriately delivers such therapies to the ventricles in response to an SVT.